[RESOLVED] Object property gets unwnated change?

I'm creating an new class object with a property that holds a generic List(of Singles). After creating this object, I change this generic list. Since I didn't pass that list as ByRef in the New-Procedure, I would expect to already created object instance NOT to change, but that not the case. Where am I wrong?

Code:

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'Class
Public Class clVessel

    Public Sub New(ByVal ID As Integer, ByVal Frequencies As List(Of Single))
        With Me
            .ID = ID
            .Frequenzen = Frequenzen
        End With
    End Sub

  Private _ID As Integer
    Public Property ID() As Integer
        Get
            Return _ID
        End Get
        Set(ByVal value As Integer)
            _ID = value
        End Set
    End Property

End Class

'in the MainForm
Public AllVessels As List(Of clVessel)
Dim Frequencies As New List(Of Single)
Frequencies.Add(300)

For j As Integer = 1 To 100
                Frequencies(0) = 300 + j
                AllVessels.Add(New clVessel(1, Frequencies))
Next

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I would have expected that all instances of clVessel in the List "AllVessels" to have different Frequencies, however all have it at the value of 400??

Not sure, but my first thought is that the value (as in byval) is the list of singles not the actual value of the singles themselves. Make sense?

Also why are you using a list of frequencies when you are only using 1 value?

List(Of T) is a class. In order to create an instance of a class you MUST invoke a constructor using the New keyword. How many times in that code do you do that for the List(Of Single)? Once, therefore there is only one object created. If there's only one List(Of Single) object then obviously every clVessel object must refer to that same object.

Passing a parameter ByVal means that whatever variable you pass is copied and the copy passed to the method. List(Of T) is a class, i.e. a reference type, so the variable contains a reference to an object. When you pass the List(Of Single) to the constructor a copy of that reference is created. As such, you have two references that both refer to the same object, but still only one object. It's always that way for reference type objects. It's exactly the same as assigning one variable to another. The second variable contains a reference to the same object. The only way to create a copy of a reference type object is to actually create a copy, which is always going to involve invoking a constructor with the New keyword again somewhere, either in your code or in the code you call.

The only difference when you pass a reference object to a sub ByVal instead of ByRef is that you can't change the original reference object. In other words you can't get a list(of single) and change that to a new list(of single).

In your case what you probably want to do is something like this:

Code:

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    Public Sub New(ByVal ID As Integer, ByVal Frequencies As List(Of Single))
        With Me
            .ID = ID
            .Frequenzen = New List(Of Single)
            .Frequenzen.AddRange(Frequencies)
        End With
    End Sub

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I didn't realize that List(of T) is an object! That was the reason for not using "New".

Thanks for the insight.

@enrico1982: The code posted was only a porting of the real code (also I tried to change the variable-names to English, but I failed!). I'm using a List in order to accept variable numbers of frequencies. That would have worked with an array, but I'm trying to use more and more of the ".Net"-Stuff!

Quote:

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Originally Posted by opus

I didn't realize that List(of T) is an object! That was the reason for not using "New".

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But you did use New: :)

Quote:

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Originally Posted by opus

Code:

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Dim Frequencies As New List(Of Single)
Frequencies.Add(300)

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You got me:thumb: